1. Field of the Invention
This invention relates to heating circuits. More particularly, but not exclusively, the invention relates to heating circuits for electric blankets or pads.
2. Description of the prior art
It is known to incorporate in an electric blanket a cable comprising a heating conductor, a sensor conductor and separating means that separates the two conductors and which has an impedance that falls with increasing temperature and/or which will melt in the event of overheating to allow contact between the two conductors. An AC power supply is connected across the heating conductor to heat it. Means is provided responsive to the impedance of the temperature sensitive means dropping to a value indicating overheating of the cable (which value will be substantially zero in the event of melting resulting in contact of the conductors) to prevent current flowing through the heating conductor. A disadvantage of such an arrangement is that the reliable detection of overheating becomes difficult due to the voltage gradient occurring along the heated conductor due to the fact that the supply voltage is applied across its ends. While this does not create a problem if the overheating is general, that is to say if it is present along the whole length of the cable, a difficulty arises if the overheating is localised at a position along the length of the cable, because the voltage at the position where the two conductors are connected by the reduced or zero impedance will be anywhere between the full supply voltage and zero, depending on the location of the position.
Prior attempts to solve this problem (see, for example, UK Pat. No. 793 783 and U.S. Pat. No. 2,831,099) have involved the use of complex and expensive circuitry and/or electromechanical components.
It is an object of this invention to provide a heating circuit as generally set forth above in which the above-mentioned problem of detecting both general and localised overheating is solved in a simple and reliable manner.